The calibration of mass standards is presently performed on conventional mechanical balances wherein masses having an unknown weight are placed on one end of a knife edge supported balance beam and standard weights are mounted on the opposite end of the beam. The slow balance response time and the handling precautions required to protect the knife-edge bearings make this a time consuming process, requiring approximately fifteen minutes to make a precise mass comparison. The calibration time becomes excessive when large numbers of accurate mass comparisons must be made on a daily basis, such as in state weights and measures laboratories.
To overcome the disadvantages experienced in the calibration of mass standards employing the conventional mechanical balances, it has been proposed to use a strain gauge load cell which produces an electric signal in proportion to the force exerted on the cell, the output of the cell being connected to electronic measuring and recording equipment and a computer to thereby automate the measurement process. It has been found that the time required to make a precise mass comparison employing a load cell is less than one-quarter the time required when using a conventional mechanical balance. In addition, measurements can be made more accurately when a large mass difference exists between the test weight and the standard weight.
The load cell mass comparator of the present invention is an improvement on known load cell mass comparators in that its precision, accuracy, repeatability, reliability and range are improved in such a manner to expand its use from a strictly laboratory instrument to a rugged dependable commercial instrument.
The load cell mass comparator of the present invention comprises, essentially, a temperature and pressure compensated load cell connected at its upper end to a floating plate through a universal joint assembly. The floating plate is slidably mounted on a plurality of guide rods extending between fixed upper and lower plates. A variable capacity spring assembly and shock absorbers are mounted between the upper fixed plate and the floating plate. The lower end of the load cell is connected to the mass to be calibrated through a universal joint, thrust bearing, and spherical load stop bearing, and a shell extends between the fixed plates to enclose the components of the comparator.